Auxiliary contact assembly



March 20, 1951 R. B. IMMEL AUXILIARY CONTACT ASSEMBLY 2 Sheets-Sheet 1 Filed Oct. 12, 1948 a INVENTOR Hal B. [mmel ATTORNEY WITNESSES: Elm Am/ u, All

March 20, 1951 R. B. IMMEL 2,546,001

AUXILIARY CONTACT ASSEMBLY Filed Oct. 12, 1948 2 Sheets-Sheet 2 INVENTOR Ralph E. fmmel.

Mam

ATTOR NEY Patented Mar. 20, 1951 AUXILIARY CONTACT ASSEMBLY Ralph B. Immel, Wilkinsburg, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application October 12, 1948, Serial No. 54,098

19 Claims. 1

My invention relates to electromagnetic contactors and more particularly to auxiliary contact assemblies or interlocks associated .with such contactors.

One broad object of my invention is the provision of auxiliary contact assemblies or interlocks for electromagnetic contactors that shall be simple, reliable, and above all considerably cheaper and also more flexible in use.

Another broad object of my invention is the provision of an auxiliary contact assembly, for electromagnetically actuated contactors, which is useful either as a normally open contact or a normally closed contact without requiring any additional parts.

A still further object of my invention is the provision of an auxiliary contact assembly, for an electromagnetically operated contactor, that may be readily adjusted in its operation both with reference to the time of operation of the main contactor as well as an adjustment for either normally open operation or normally closed operation, or both of such adjustments.

Other objects and advantages will become more apparent from a study of the following specification and the accompanying drawings forming part of my disclosure and in which drawings: 7

Figure 1 is a side view of my auxiliary contact assembly showing the parts arranged for normally Open operation and showing the apparatus in such a position that the main contacts of the magnetic contactor are in the open position;

Fig. 2 is a view similar to Fig. l, but showing the elements in closed position;

Fig. 3 is a longitudinal sectionalview of main portions of the auxiliary contact assembly as arranged in Fig. 1;

Fig. 4 shows a detail;

Figs. 5 and 6 are side views of the operating arm for the auxiliary contact assembly, with the actuating element shown in two positions of adjustment in the two views;

Fig. 7 is an enlarged perspective view in vertical section of the upper end of the operating arm showing in Fig. 5;

Fig. 8 is a view similar to the showing in Fig, 1 but with the auxiliary contact assembly arranged for normally closed operation;

Fig. 9 is a showing of the subject matter of Fig. 8 but showing the elements in closed position;

Fig. 10 is a longitudinal sectional view of the main portions of the auxiliary contact assembly as arranged in Fig. 8, and

Fig. 11 is a plan view of a modified contact and terminal connector, showing the part in position on a broken-out portion of its mounting of insulating material.

In Figures 1, 2, 8 and 9, the base I comprises the metal or other backing of the electromagnetic contactor, not shown as a whole. To this base I is secured the support 2 of insulating material. This support carries the main portions of the auxiliary contact assembly.

Support 2 is mounted by means of suitable screws or bolts 33 and 34 to the forward side of the main contactor, or electromagnetic contactor. This main contactor has an arm 3 secured to its armature, not shown. Actuation of the main contactor causes the contactor arm 3 to swing counterclockwise upon closing of the main contactor and clockwise, by gravity or by spring bias upon release of the armature.

The arm 3 carries a laterally projecting bar 4 to which the auxiliary contact operating arm I4 is rigidly secured. The securing means comprise the bolts 5, look washers 8 and the nuts I3.

The upper end of the operating arm i4 is provided with a hexagonal recess 24. Into this recess the hexagonal plunger I5 is disposed in fitting relation. The forward surface of the plunger has an annular recess, as shown, for receiving the compression spring [6. The plunger IS in effect constitutes the head of the bolt 25. The spring It is disposed about the bolt and between the bottom of the recess 24 and the bottom of the annular recess in the plunger. The threaded end of the bolt 25 is provided with the nut l'i. This nut H is also hexagonal and normally fits into a matching hexagonal recess at the upper forward surface of arm l4. The rearward pro: jection of the plunger [5 may be adjusted to take the positions shown in either Fig. 5 or 6 or any intermediate position. The adjustment of plunger I5 is effected by merely depressing the plunger l5 against the compressive force of the spring I6. This operation causes nut I! to move out of its recess. This nut may then be turned to any position within the adjustment range provided. After the adjustment is made, the plunger I5 is released so that nut ll falls into its recess. The spring it thus holds all parts in the desired position of adjustment.

The auxiliary contact assembly includes two blocks and 4| of insulating material. The block 40 may be considered U-shaped with the U being very shallow. The block 4| is distinctly U-shaped as is readily apparent from Figs. 1, 2, 3, 8, 9 and 10.

These blocks are so disposed in relation to each other that the ends of the legs as 42 and 43, abut against each other and legs 44 and 45 abut against each other as shown to form a contact chamber between the legs, as shown.

The end surface of each leg is provided with a shallow groove 46 rectangular in shape at each end, and bulged out to a generally circular shape at its middle, all as more clearly apparent from the showing in Fig. 11. The outward side surfaces of the legs 43 and 45 are each provided 3 with a rectangular notch 41 for receiving the body portion 48 of the contact strips 49 having the contact button 50 on its inner arm and the terminal connector 52 on the outer arm 53.

The middle or generally circular portion of the contact strip is provided with a generally circular hole 54 disposed to register with the holes 55 and 56 disposed axially of each of the legs 42 and 43, and it and 45. In mounting the two U-shap'e'd blocks of insulation 40 and M, the bolts IQ are disposed in the holes 54, 55 and 56. The grooves 46 in each end surface is selected to have a depth about one-third the thickness of the contact strips 49. This means when the nuts I9 are tightened to firmly hold the blocks 48 and 4| on the support 2, the contact strips 49 are firmly gripped in the grooves 4S. This will be readily apparent from a study of Figs. 1, 2, 8 and 9.

The block 46 is, at its outer mid region provided with a cylindrical projecting boss 69 and the block 4| is provided with a similar boss 6| in a similar position. The block 4| is provided with a cylindrical boss 62 at the base of a transverse slot in the bight region of the U-shaped block 4|. These bosses 6% 6| and 52 provide seats for compression springs S and 59.

The blocks 40 and 4| are provided with rectangular holes axially of the bosses 6| 6| and 62 for receiving the rectangular strip 63. This stri 63 is provided with a pair of arms 64 projecting from the edges. A contact button bridging member 65 having a spring receiving boss 66 at its rear mid region and a pair of contact buttons at the front ends for coaction with the contact buttons 50.

The rectangular member 63 fits fairly loosely into the rectangular openings or holes axially of the bosses 60, BI and 52. The boss 68 is also provided with a rectangular opening axially of the boss to provide a fairly loo'se mounting of the member 65 on the rectangular strip '63.

The front end 6? of the strip 53 is provided with a transverse hole for receiving the pin 1. A saucer shaped spring stop 6 is thus held in position'on the strip 63. To mount the pin 1, the stop 5 is moved axially against the spring bias until the axis of the transverse hole in the end filc'lears the stop 6 and then the pin 1 is inserted. When the spring 9 moves the stop 6 to the position shown, then the pin 1 is prevented from falling out by the cylindrical side walls of the stop 6.

In Fig. 1 block 49 is mounted adjacent the support 2 with the contact strips 49 in the position, namely, so that the contact buttons at the inner ends of the strips 49 face the front, or left, as seen in Fig. 1. The bridging member 65 is disposed on the strip 63 to the right, or front, side of the arms 64. Spring 59 is disposed in the position shown and thus biases the bridging member E5 against the arms 54. The spring 9 is selected to have a larger force than spring 59. The result will be that the bridging member 65 will be remote from the contact buttons on strips 49, and a normally open interlock is provided for the main contactor.

When the main contactor is operated the arm I4 is moved counterclockwise with the result that plunger l5 engages the end 61'. The time of such engagement with reference to the full movement of arm i4 is determined by the plunger adjustment on arm it. The operation of strap $3 axially compresses spring 9. Spring 59 is'e'xpanded, but what is more important the bridging member 4 65 interconnects the contact straps 49 as shown in Fig. 2.

When it is required to change the interlock to a normally closed contact, then block 4| is mounted adjacent the support 2. The elements, comprising the projecting arms 64, contact strips 39, spring 59, bridging member 65, blocks 4|] and 4|, and strap 63, all remain in the same relative position but the spring 9 is shifted to the other endof strap '63 Elements B and 'l are also so shifted. The spring 9 is then disposed between the back of the saucer shaped spring stop 6 backed by in 1 and the boss 6|! now facing the front.

The terminals 52 are "also turned on the contact strip to make the head of the screw in each of the terminals face forward.

Upon operation of the arm M counterclockwise the plunger I5 again moves the strap d3 axially but now the arms 64 move the bridging member 6'5 from the position shown in Fig. '8 to the position shown in Fig. 9.

To facilitate the mounting of my interlock for either a make interlock or a break interlock, I place the abbreviated word Norm. on the flat surface Hi and the word'Open on thesurface T l. On surface 12 I place the word Norm, not shown, and on surface 1-3 the word Closed, also not shown.

With the parts thus identified, it is apparent that change from one type of interlock to another type is greatly facilitated and the chance of error is reduced to substantially nothing.

I am well aware that many types or interlocks are known in the trade, but I have not been aprised of any interlock utilizing so few relatively standard parts, and parts that can be so readily interchanged. All of the auxiliary contact or interlock assemblies of the prior art known to 'me were assembled specifically either for normal open operation or for normal closed operation. Once an auxiliarycontact unit had been assembled, it could not be converted from a normally open to a normally closed assembly, as the parts for each assembly were not the same.

The prior art devices were thus expensive and not flexible in use, because it was necessary to make and stack a large numberbf parts. This was not only a disadvantaget'o the manufacturer but proved very inconvenient to the user. In general it was easier from an operational point of view to "reso'rtto the expense of buying acomplete new assembly each time a change-over was required, or a repair was needed.

While I have shown but one main embodiment and some details, I do not Wish to be limited by the detail'shown and described but only'such limitations are to be considered as are embodied in the'clai'ms hereto appended.

I claim as my invention:

1. In an electrical interlock, in combination, a base, a support of insulating material mounted on the base, a U-shaped member disposed with its outer bottom of the U against the support, a second U-shaped member having its legs disposed in aligned relation to the first U-shaped member, contact strips between the aligned legs ending in contact buttons in the region between the legs, said U-shaped members being provided with aligned openings in the bight's'of each U, a rigid strap mounted loosely but non-'rotatably in said opening so that said strap ma be moved longitudinally, said strap being provided with projecting stops, a contact button bridging bar mounted on the'str'ap, spring means mounted on the strap between the inner bight of the second U-shaped member and said bar to bias said bar against said projecting stops, spring retainin means near the outer end of the strap, and spring means disposed on the strap between said spring retaining means and the outer surface of the bight of the second U-shaped member for biasing said strap to its outer, or most forward, position to thus hold said bar remote from the contact buttons.

2. In an electrical interlock, in combination, a base, a support of insulating material mounted on the base, a U-shaped member disposed with its outer bottom of the U against the support, a second U-shaped member having its legs disposed in aligned relation to the first U-shaped member, contact strips between the aligned legs ending in contact buttons in the region between the legs, said U-shaped members being provided with aligned openings in the bight of the U, a rigid strap mounted loosely but nonrotatably in said opening so that said strap may be moved longitudinally, said strap being provided with projecting stops, a contact button bridgin bar mounted on the strap, spring means mounted on the strap between the inner bight of the second U-shaped member and said bar to bias said bar against said projecting stops, spring retaining means near the outer end of the strap, and spring means disposed on said strap between said spring retaining means and the outer surface of the bight of the second U-shaped member for biasing said strap to its outer, or most forward position" to thus hold said bar remote from the contact buttons.

3. In an electrical interlock for an electromagnetic contactor, in combination, a support for said interlock secured to the contactor, a shortlegged U-shaped member, a long-legged U-shaped member, said members being disposed toward each other so that the open ends of the Us face each other and the legs are in alignment, contact strips between the ends of the legs, means for firmly mounting the U-shaped members, in the mentioned relation to each other, on said support with the inner ends of the contact strips being disposed in aligned spaced relation in the space between the legs and forming contact buttons, a strap mounted for axial movement in aligned apertures in the bights of the U-shaped members, a contact bridgin bar mounted on the strap, and spring means on the strap for biasing said bar to one position with reference to said contact buttons.

4. In an electrical interlock for an electromagnetic contactor, in combination, a support for said interlock secured to the contactor, a shortlegged U-shaped member, a long-legged U-shaped member, said members being disposed toward each other so that the open ends of the Us face each other and the legs are in alignment, contact strips between the ends of the legs, the ends of said legs being provided with grooves having, for each pair of aligned legs, a combined depth less than the thickness of said contact strips, means for firmly mounting the U-shaped members, in the mentioned relation to each other, on said support to thus firmly hold said contact strips in said grooves with the inner ends of the contact strips being disposed in aligned spaced relation in the space between the legs and forming contact buttons, a strap mounted for axial .movement in aligned apertures in the bights of the. U-shaped members, a contact bridging bar mounted on the strap, and spring means on the 6 strap for biasing said bar to one position with reference to said contact buttons.

5. In an electricalinterlock for an electromagnetic contactor, in combination, a support for said interlock secured to the contactor, 'a short-legged U-shaped member, a long-legged U-shaped member, said members being disposed toward each other so that the open ends of the Us face each other and the legs are in alignment, contact strips between the ends of the legs, means for firmly mounting the U-shaped members, in the mentioned relation to each other, on said support with the inner ends of the contact strips being disposed in aligned spaced relation in the space between the legs and forming contactbuttons, a strap mounted for axial movement in aligned apertures in the bights of the U-shaped members, a contact bridging bar mounted on the strap, and spring means on the strap for biasing said bar to one position with reference to said contact buttons.

6. In an electrical interlock for an electromagnetic contactor, in combination, a support for said interlock secured to the contactor, a short-legged U-shaped member, a long-legged U-shaped member, said members being disposed toward each other so that the open ends of the Us face each other and the legs are in alignment, contact strips between the ends of the legs, the ends of said legs being provided with grooves having, for each pair of aligned legs, a combined depth less than the thickness of said contact strips, means for firmly mounting the U-shaped members, in the mentioned relation to each other, on said support to thus firmly hold said contact strips in said grooves with the inner ends of the contact strips being disposed in aligned spaced relation in the space between the legs and forming contact buttons, a strap mounted for axial movement in aligned apertures in the bights of the U-shaped members, a contact bridging bar mounted on the strap, and spring means on the strap for biasing said bar to one position with reference to said contact buttons.

7. In an electrical interlock for an electromagnetic contactor. in combination, a support of insulating material mounted in fixed relation to the contactor, a relatively shallow U-shaped member of insulating ,material, a relatively deeper U-shaped member of insulating material,

contact strips being bulged into circular shape in,

the region of the circular portion of the groove, each of said contact strips being provided with a circular hole in the axis of the circular bulged portion, retaining bolts disposed through the holes axially of the legs and the holes in the contact strips and engaging the support to firmly hold the U-shaped members and the contact strips on the support, said contact strips being provided with spaced contact buttons in. the region between the legs and beingprovided with terminal connectors at the outer ends, said U-shaped members being provided with rectangular aligned holes through the bights, an elongated strap disposed loosely in the rectangular holes for axial movement in the rectangular p holes, and a contact bridging bar on the strap in the region between the legs.

8. In an electrical interlock for an electromagnetic contactor, in combination, a U-shaped member, a contact strip mounted in the end face of one leg of the U-shaped member and having a contact button facing the bight of the. U, a similar contact strip in the end face of the other leg and having a similar contact button similarly disposed, said buttons being thus disposed in spaced relation between the legs, a contact bar for bridging the contact buttons, a strap disposed for axial movement in an aperture in the bight of the U, said strap having transverse projections in. the space between the legs and below the contact buttons, said bar being disposed on the strap below the transverse projections, a spring be tween the bar and the bottom of the bight of the. U biasing said bar against said transverse projections, a spring stop on the end of the strap below the U, and a spring between said stop and the outer bottom of the U biasing said strap and bar toward the inner bottom of the U.

9. In an electrical interlock for an electromagnetic: contactor in combination, a U -shaped memher, a contact strip mounted in the end face of one leg of the U-shaped member and having a contact button facing the bight of the U, a similar contact strip in the end face of the other leg and having a similar contact button similarly disposed, said buttons being thus disposed in spaced relation between the legs, a contact bar for bridging the contact buttons, a strap disposed for axial movement in an aperture in the bight of the U, said strap having transverse projections in the space between the legs and below the contact buttons, said bar being disposed on the strap below the transverse projections, a spring between the bar and the bottom of the bight of the U biasing said bar against said transverse projections to move the bar to bridge said contact buttons, a bridge bar secured to the ends of the legs having an opening for receiving the end of the strap projecting from the open end of the U, a spring stop on the end of the strap remote from the open end of the 'U, and a spring between the stop and the bridge bar for biasing the strap and thus the contact bar to bridge the contact buttons.

10. In an electrical interlock for an electromagnetic contactor, in combination, .a U-shaped member, a contact strip mounted in the end face of one leg of the U-shaped member and having a contact button facing the bight of the U, a similar contact strip in the end face of the other leg and having a similiar contact button similar- 1y disposed, said buttons being thus disposedv in spaced relation between the legs, a contact bar for bridging the contact buttons, a strap disposed for axial movement in an aperture in the bight of the U, said strap having transverse projections in the space between the legs and below the contact buttons, said bar being disposed on the strap below the transverse projections, a spring between the bar and the bottom of the bight of the U biasing said bar against said transverse projections to move the bar to bridge said contact buttons, a bridge bar secured to the ends of the legs having an opening for receiving the end of the strap pro-- jecting from the open end of the U, a spring stop on the end of the strap remote from the open end of the U, and a spring between the stop and the bridge bar for biasing the strap and thus the contact bar to bridge the contact buttons.

11. In an electrical interlock for an electromagnetic contactor, in combination, a U-shaped 8. member, 'a' contact :s'trip mountedin the end face of one leg of the U-shaped member and having a contact button facing the bight-oi the U, asimilar contact strip in the, end face of the other lee and having a similar contact button similarly disposed, said buttons being thus disposed in spaced relation between the legs, a contact bar for bridging the contact buttons, a strap disposed for axial movement in an aperture in the bight or" the U, said strap having transverse projections in the space between thelegs and below the contact buttons, said bar being disposed on the strap below the transverse projections, a spring between the bar and the bottom of the bight of the U biasing said bar against said transverse projections to move the bar to bridge said contact buttons, a bridge bar secured to the ends of the legs having an opening for receiving the end of the strap projecting from the open end of the U, a spring stop on the end of the strap remote from the open end of the U, and a spring between the stop and the bridge bar for biasing the strap and thus the contact bar to bridge the contact buttons.

12. In an electrical interlock for an electromagnetic contactor, in combination, a U-shaped member, a contact strip mounted in the end face of one leg of the U-shaped member and having a contact button facing the bight of the U, a similar contact strip in the end face. of the other leg and having a similiar contact button similarly disposed, said buttons being thus disposed in spaced relation between the legs, a contact bar for bridging the contact buttons, a strap disposed for axial movement in an aperture in the bight of the U, said strap having transverse projections in the space between the legs and below the contact buttons, said bar being disposed on the strap below the transverse projections, a spring between the bar and the bottom of the bight of the U biasing said bar against said transversepro-jections to move the bar to bridge said contact buttons, a bridge bar secured to the ends of the legs having an opening for receiving the end of the strap pro jecting from the open end of the U, a spring stop on the end of the strap remote from the open end of the U, and a spring between the stop and the bridge bar for biasing the strap and thus the contact bar to bridge the contact buttons.

13. In an electrical interlock for an electromagnetic contactor, in combination, a U-shaped member, a contact strip mounted in the end face of one leg of the U-shaped member and having a contact button facing the bight of the U, a similar contact strip in the end face of the other leg and having a similar contact button similarly disposed, said buttons being thus disposed in spaced relation between the legs, a contact bar for bridging the contact buttons, a strap disposed for axial movement in an aperture the bight of the U, said strap having transverse projections in the space between the legs and below the contact buttons, said bar being disposed on the strap below the transverse projections, a spring between the bar and the bottom of the bight of the U biasing said bar against said transverse projections, a spring stop on the end of the strap below the U, and a spring disposed between said stop and the outer bottom of said U biasing said strap and bar toward the inner bottom of the U.

14. In an electrical interlock for an electromagnetic contactor, in combination, a U-shaped member, a contact strip mounted in the end face of one leg of the U-shaped member and having a contact button facing the bight of the 'U, a similar contact strip in the end face of the other leg and having a similar contact button similarspaced relation between the legs, a contact bar for bridging the contact buttons, a strap disposed for axial movement in an aperture in the bight of the U, said strap having transverse projections in the space between the legs and below the contact buttons, said bar being disposed on the strap below the transverse projections, a spring between the bar and the bottom of the bight of the U biasing said bar against said transverse projections, a spring stop on the end of the strap below the U, and a spring disposed between said stop and the outer bottom of said U biasing said strap and bar toward the inner bottom of the U.

15. In an electrical interlock for an electromagnetic contactor, in combination, a U-shaped member, a contact strip mounted in the end face of one leg of the U-shaped member and having a contact but-ton facing the bight of the U, a similar contact strip in the end face of the other leg and having a similar contact button similarl disposed, said buttons being thus disposed in spaced relation between the legs, a contact bar for bridging the contact buttons, a strap disposed for axial movement in an aperture in the bight of the U, said strap having transverse projections in the space between the legs and below the contact buttons, said bar being disposed on the strap below the transverse projections, a spring between the bar and the bottom of the bight of the U biasin said bar against said transverse projections, a sprin stop on the end of the strap below the U, and a spring disposed between said stop and the outer bottom of said U biasing said strap and bar toward the inner bottom of the U.

16. In an electrical interlock, in combination, a base, a support mounted on the base, an elongated member of insulating material disposed on the Support so that one side is against the support, said member having raised flat portions at each end on the side opposite to the support, a second somewhat similar elongated member having similar raised fiat portions, so disposed on the first member that the flat end portions are contiguous, contact strips between the contiguous surfaces of the fiat portions ending in contact buttons in the region between the mid-portions of the two members, aligned openings in the midportions of the members, a strap mounted for longitudinal motion but non-rotatably in the openings, said strap being provided with shoulders, a contact bridging bar mounted on the strap, spring means mounted to bias the contact bar against the shoulders, spring retaining means between the outer end of the strap and the outer surface of the second member, and spring disposed on the strap between the spring retaining means and the outer surface of the second member for biasing said strap to a position to hold the bar remote from the contact buttons.

1'7. In an electric interlock, in combination, a base, an inner arcuate member of insulating material mounted on the base with the concave portion of the are facing away from the base, a somewhat similar outer arcuate member, disposed in an opposite sense with reference to the base, on the first member, contact straps disposed between the registering ends of the member projecting inwardly to form spaced aligned cont-act .buttons between the members, aligned openings in the members, the alignment being transverse of the aligned contact buttons, a. strap disposed for slidable non-rotary movement in the open ings, said strap having projecting lug near the middle thereof, a contact bar for bridging the contact buttons provided with a transverse opening for receiving the strap, a biasing spring being selectively positionable in the order named either between the lugs on the strap and the outer arcuate member, or between the lugs on the strap and the inner arcuate member, spring means engaging the strap to bias the strap outwardly.

18. In an electric interlock, in combination, a base, an elongated block of insulating material thicker at each end than in the middle mounted on the base, a second somewhat similar elongated block of insulating material mounted on the first member, the members by reason of their thicker ends being shaped to have end-engagement with each other at corresponding ends and to form a space between the members at generally the midportion of the members, aligned openings at the mid-portions, contact straps disposed between the engaging ends projecting into the space to form a pair of spaced contact buttons, a strap mounted for slidable motion in the openings, said strap having transverse projections and being long enough that the ends of the strap project beyond the non-contiguous surfaces of the members with the transverse projections disposed in the space between the members, a contact brid in bar disposed on the strap adjacent the projections, a spring coacting with one of the bridging members and the contact bridging bar to bias the bar against the transverse projections, and spring means for biasing the strap toward one end position.

19. In an electric interlock, in combination, a base, an elongated block of insulating material thicker at each end than in the middle mounted on the base, a second somewhat similar elongated block of insulating material mounted on the first member, the members being shaped by reason of their thicker ends to have end-engagement with each other at corresponding ends, and to form a space between the members at generally the mid-portion of the members, aligned openings at the mid-portions, contact straps disposed between the engaging ends projecting into the space to form a pair of spaced contact buttons, a strap mounted for slidable motion in the openings, said strap having transverse projections and being long enough that the ends of the strap project beyond the non-contiguous surfaces of the members with the transverse projections disposed in the space between the members, a contact bridging bar disposed on the strap adjacent the transverse projections, a spring disposed about the strap and coacting with one of the bridging members and the contact bridging bar to bias the contact bridging bar against the transverse projections, spring retaining means at the outer end of the strap, and a sprin between the spring retaining means and the outer bridging member to bias the strap outwardly.

RALPH B. IMMEL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,513,943 Soreng Nov. 4, 1924 1,598,932 Pettus Sept. 7, 1928 

